Common fibre optic fusion splicing problems – causes and solutions

Fibre optic fusion splicing problems rarely begin when the arc is fired. A poor cleave, contaminated fibre, a bubble, high loss or a splice that breaks while the protector is fitted usually has an earlier or later cause. The source is most often fibre preparation, tool condition, program selection, splice protection or routing in the tray. This guide helps you identify the symptom, find the most likely cause and decide whether the splice should be remade immediately.

The biggest problems in fibre optic fusion splicing

Problem	What usually happens	Effect on the job
Poor cleave or contamination	Angled or chipped end face; coating, gel, dust or IPA residue	The most common reason for rejecting an end and repeating strip–clean–cleave
Cable and closure preparation	Removing jackets and gel, securing the cable, identifying tubes and planning slack	The largest overall time cost
Protector fitting and splice transfer	Forgotten protector, breakage during transfer or the wrong oven profile	Loss of a good splice and a complete restart
Dirty or uncalibrated splicer	Contamination in V-grooves, clamps or optics; worn electrodes	Unstable arc, poor alignment and false alarms
Wrong program or dissimilar fibres	Mismatched mode, different mode-field diameter or fibre design	Higher attenuation that another attempt may not remove
Routing in the tray	Radius too small, pressure from the holder, twisting or insufficient slack	Loss appears only after routing or closing the closure
Incorrect test interpretation	Splicer estimate treated as a measurement, or one-way OTDR testing	Unnecessary resplicing of a good joint or acceptance of a bad one

1. Poor cleaves and contamination — the most common causes of rework

The cleaved surface should be flat, clean and free from chips. Common causes of trouble include a worn or incorrectly positioned cleaver blade, dirty clamping pads, acrylate residue after stripping, touching the prepared end, and dust or gel carried over from the cable. A cut that is too deep can leave a microcrack, while one that is too shallow can produce an irregular break.

In field data reported by Sumitomo, around 15% of prepared ends were rejected because of an incorrect cleave. That is one problem for roughly every six or seven attempts. In the same study, a correct work cycle took about 60 seconds, while re-preparing a poor cleave extended it to around 80–100 seconds. This is a result from one manufacturer’s system, not a global statistic, but it illustrates the scale of rework.

According to ITU-T L.400, the typical cleave angle for a single fibre should be less than 1°. The requirements of the splicer and cleaver manufacturers, and the operator’s procedure, should always take precedence.

2. Troubleshooting table: symptom, cause and action

Symptom	Most likely causes	What to check
Large Cleave Angle / Cleave Shape NG	Dirty or worn cleaver, poor technique, damaged end face; sometimes dirty splicer optics	Clean the cleaver and clamps, check blade position and prepare the ends again
Bubble in the glass	Contaminated end face, coating residue, poor cleave, incorrect prefuse or unstable arc	Break the joint, remove the cause and make a new splice
Bulge	Excessive overlap or incorrect arc current or duration	Check the program, calibration and electrode condition; remake the splice
Neck-down	Insufficient overlap, excessive heating or incorrect feed	Check the parameters and prepare both ends again
Core offset	Contamination in the V-grooves, incorrect seating, different diameters or loose-tube slippage	Clean the grooves and clamps, reseat the fibres and confirm the fibre type
Not fused / separation	Arc too weak, prefuse too short, incorrect gap or electrode problem	Run arc calibration and check the program and electrodes
High Estimated Loss	Poor end face, contamination, bubble, incorrect mode, dissimilar fibres or dirty objective lens	Do not decide from the estimate alone; remove any visible defect and verify the result by measurement
Splice breaks during proof test or removal	Micro-nick from the stripper, weak splice or uneven tension	Check the stripper and handling method; prepare and splice both ends again
Loss increases after placement in the tray	Microbend or macrobend, overly tight holder, hot protector or pressure from the cover	Remove the splice, straighten the route, check the holder and bend radius, then decide whether resplicing is necessary
Good image, poor OTDR result	Different MFDs, a real defect not visible in the image, bending or a one-way test error	Check the fibre types and measure in both directions

3. Problems with splice protectors

A splice can be correct when it leaves the splicer and still be damaged during protection. The protector, oven, transfer and tray holder should therefore be treated as one process.

Before splicing

• Forgetting the heat-shrink protector usually means breaking a good splice and starting again.
• The wrong length or diameter does not fit the prepared fibre or the tray holder.
• A protector contaminated with gel, dust or moisture may recover unevenly.
• A strip length that does not match the protector design leaves bare glass unprotected or places coating in the splice zone.

During transfer and heating

• the fresh splice breaks when the clamps are opened or the protector is moved;
• the splice is not centred over the reinforcing member;
• the fibre is twisted or bent in the uncoated zone;
• incorrect time or temperature causes incomplete recovery, bubbles or a melted buffer;
• the protector sticks to the oven surface;
• the oven program is intended for a different length or construction.

During cooling and tray routing

• a protector that is still soft becomes permanently bent when placed in the tray;
• an overly tight holder compresses the protector and introduces a microbend;
• a protector that is too small in diameter slips out of the holder;
• the cover, an adjacent fibre or an enclosure component presses on the splice;
• the technician presses the centre of the protector instead of its ends.

4. When should a splice be remade immediately?

Remake the splice when you see an unambiguous geometric or mechanical defect:

• a bubble or inclusion in the glass;
• a clear core offset;
• incomplete fusion or fibre separation;
• a bulge, neck-down or visible kink;
• failure during the proof test;
• a splice damaged during transfer;
• bare glass outside a correctly fitted protector;
• measured loss above the project or operator limit;
• measurable reflection at the splice where the procedure does not allow it.

When should you diagnose before cutting?

• the splicer reports high estimated loss but the image shows no definite defect;
• the OTDR shows a “gainer” or very different values depending on direction;
• old and new fibres, or fibres with different MFDs, are being joined;
• attenuation appeared only after the protector was placed in the tray;
• cleave warnings recur despite correctly prepared ends — dirty optics may be responsible;
• a visible line results from different fibre structures rather than a gap in the splice.

The splicer’s loss estimate is an image assessment, not a transmission measurement. With dissimilar fibres, a one-way OTDR test can show excessive loss or an apparent gain. The most reliable value for a singlemode splice is the average of measurements taken in both directions. Acceptance thresholds always come from the power budget, project documentation and operator requirements.

5. What takes fibre splicers the most time?

The arc is only a small part of the complete job. In a large closure, most time is spent on:

1. gaining access to the cable, chamber, pole or cabinet;
2. opening the cable and removing gel;
3. securing the cable and preparing the closure;
4. identifying tubes, colours and fibre pairs to be joined;
5. planning fibre length and sequence;
6. repeating stripping, cleaning and cleaving;
7. heating and cooling protectors;
8. routing slack without crossings or overly tight bends;
9. testing, interpreting traces and completing documentation;
10. reopening the closure when a problem is found only after it has been closed.

In repetitive work, the oven can become a bottleneck, especially when the machine has only one heater. A well-organised technician can prepare the next fibre while the protector cycle is running. Preventing rework still saves far more time than shortening the arc by one second.

6. Cleanliness, calibration and field conditions

When several consecutive splices show the same fault, do not repeat the same operation indefinitely. Stop and find the common cause. Check the cleaver, stripper, V-grooves, clamps, optics, electrodes, selected program and arc calibration in sequence.

Temperature, humidity and atmospheric pressure affect the arc. Field work also brings wind, dust, rain, condensation, frost, an unstable work surface and limited space. Cold fibre may be more brittle, IPA evaporates more slowly and the optics may fog. After a change in altitude, weather, electrodes or work location, carry out the checks and calibration specified by the splicer manufacturer.

7. Checklist before, during and after splicing

Before starting

• confirm the fibre types and correct program;
• check the available slack and tray-routing plan;
• match the protector to the fibre, holder and oven;
• slide on the protector before stripping;
• clean the cleaver, V-grooves and clamps;
• run arc calibration if required;
• prepare a sealable container for fibre offcuts.

During preparation and splicing

• do not touch exposed glass or the cleaved end face;
• do not let the prepared end touch the table, holder or protector;
• inspect the image of both ends before accepting the splice;
• if an error recurs, remove its cause instead of making more attempts;
• do not force a splice when end-face damage is visible.

After splicing

• transfer the fibre evenly, without twisting or jerking;
• centre the splice in the protector;
• use the correct oven program for the protector;
• after the cycle, keep the protector straight until it cools;
• inspect the recovery, protector ends, bubbles and fibre path;
• place the protector in the holder without pressing its centre;
• check attenuation after final routing as well;
• update labels and documentation before closing the closure.

8. How can splice-protection problems be reduced?

The protector must match the fibre diameter and construction, prepared length, oven, holder and clearance beneath the cover at the same time. A larger product does not always provide better protection: it may not fit the holder, may interfere with adjacent fibres or may force excessively tight routing.

Heat-shrink protectors are available in a wide range of lengths, diameters and rigidity levels. Where installation without an oven and protection after the splice has been made are important, the crimp ANT series can be an alternative for compatible single fibres. It requires a dedicated tool and a suitable tray, so it is not a universal replacement for heat-shrink protection.

Detailed dimensional criteria are covered in How to choose a fibre optic splice protector. Current technical data sheets and declarations are available in the Download section.

Frequently asked questions

Why does the splicer show high splice loss?

The most common causes are contamination, a poor cleave, a bubble, fibre misalignment, the wrong program, electrode condition or dirty optics. The estimate is based on the image, so interpret it together with the splice appearance and the required measurement.

Does a 0.00 dB result on the splicer guarantee a good splice?

No. It is an image-based estimate, not an actual transmission measurement. A splice may contain a defect that the algorithm cannot see, or it may be damaged later while the protector is fitted or routed in the tray.

Does every bubble in a protector mean the splice must be remade?

A bubble in the splice glass itself is a serious defect. A bubble in the protector material must be assessed according to its position and the product requirements. If it is next to bare fibre, moves the fibre or indicates incomplete closure, the protection should not be accepted.

Why does attenuation increase after the protector is placed in the tray?

The usual cause is a microbend or macrobend: the protector is still warm, the holder is too tight, the radius is too small or the cover presses on the fibre. Check the route and holder before deciding to remake the splice.

Why does the OTDR show different results from each direction?

Dissimilar fibres can have different mode-field diameters and backscatter coefficients. The OTDR may show excessive loss from one direction and an apparent gain from the other. The actual splice loss is determined by averaging the values measured in both directions.

What takes the most time in fibre optic fusion splicing?

Usually not the arc itself, but opening and preparing the cable, removing gel, identifying fibres, re-preparing poor ends, heating protectors, routing in the tray, testing and documentation.

When should a crimp protector be chosen instead of a heat-shrink protector?

A crimp protector is useful when installation must be completed without an oven or when protection needs to be fitted after the splice has been made. Compatibility with the fibre, tool and tray holder must still be confirmed. Heat-shrink protection remains available in a wider range of dimensions and constructions.

Sources

This guide was prepared from technical documentation and training materials, particularly ITU-T L.400, Sumitomo Electric NanoTune, Corning AN103, AFL/Fujikura 90R Troubleshooting, FOA Fusion Splicing and Fluke Networks — OTDR gainers. Always confirm acceptance limits, oven parameters, maintenance procedures and consumable replacement intervals in the current documentation for the equipment and project concerned.